1. Field of the Invention
The present invention relates to a rodless power cylinder having a piston disposed in a tube and moving along the axis of the tube and an external moving body disposed outside the tube and coupled to the piston through a slit formed on the wall of the tube. More specifically, the present invention relates to a rodless power cylinder provided with a bore having a non-circular cross section and the slit-side inner surface on which the slit is formed has a substantially no or a very small curvature.
2. Description of the Related Art
A rodless power cylinder includes a tube (a cylinder barrel) having an axial slit in the wall and a piston disposed in the bore of the tube and is movable along the longitudinal axis of the tube. The movement of the piston is transferred to an external moving body by a member which couples the external moving body to the piston through a slit formed on the wall of the tube along the longitudinal axis thereof. Usually, an inner seal band and an outer seal band are disposed on the inner and the outer wall surfaces of the tube along the slit in order to cover the inner and the outer openings of the slit.
Rodless power cylinders having non-circular cross sections such as elliptical cross sections or oblong circular cross sections are disclosed in various publications.
For example;
(A) Japanese Unexamined Patent Publication (Kokai) No. 50-89775 discloses a rodless power cylinder including a bore having a substantially rectangular cross section. The slit-side inner surface of the bore is formed as a flat plane, and the slit is sealed by a thin metal band which contacts the flat slit-side inner surface in face-to-face contact.
(B) Japanese Unexamined Utility Model Publication (Kokai) No. 1-104407 and Japanese Unexamined Utility Model Publication (Kokai) No. 1-180001 discloses rodless power cylinders having non-circular bores. In the rodless power cylinders of these publications, the openings of the slits on the inner surface of the bore are widened in order to form grooves for accommodating elastomer seal bands. The elastomer seal bands fitted into the grooves have a relatively large thickness.
(C) Japanese Unexamined Patent Publication (Kokai) No. 62-46009 discloses a rodless power cylinder having a circular cross section bore. In the rodless power cylinder, though the bore has a circular cross section, the portion of the inner surface of the bore on both sides of the slit is formed as a recess having a curvature larger than the curvature of the surface of the bore. A thin inner seal band having a curvature matching the curvature of the recess is used to seal the opening of the slit.
(D) Japanese Unexamined Patent Publication (Kokai) No. 54-28978 discloses a rodless power cylinder having a tube with a substantially circular cross section. The whole slit-side inner surface of the bore has a curvature smaller than the outer wall of the tube and the recess is not formed on the slit-side inner surface at the portion of the slit opening. The inner seal band is formed as a thin flat belt and is deflected into the slit by the internal fluid pressure of the bore. The internal pressure is sealed by the contact between the transverse edges of the seal band and the slit-side inner surface.
(E) Japanese Unexamined Patent Publication (Kokai) No. 56-124711 discloses an arrangement of the piston packing disposed at piston ends of a rodless power cylinder. The piston packing in this publication is formed as an annular shape and has an outer lip contacting with the inner surface of the bore. The portion of the periphery of the outer lip has a complementary shape of the inner surface of the seal band.
(F) Japanese Unexamined Utility Model Publication (Kokai) No. 1-180001 discloses another type of piston packing. The piston packing in this publication is also formed as an annular shape and has an outer lip. However, in this publication, bridges connecting the outer lip and an inner lip (a base portion) of the packing are provided at the portions corresponding to the edges of the inner seal band. These bridges increase the force for urging the edges to the inner surface of the bore in order to increase the sealing capability of the seal band.
(G) Japanese Unexamined Patent Publication (Kokai) No. 1-6505 discloses an arrangement of dampers for receiving the piston at its stroke ends. In this publication, the dampers are attached to end members which close both ends of the tube. The end members are provided with holes on the faces opposing the piston ends for fitting rod-shaped rubber dampers. The rod-shaped rubber dampers have stepped diameter portions and are fitted to the end members by inserting the larger diameter ends into the holes on the end members. When the piston hits the smaller end of a damper at its stroke end, the rubber damper resiliently deflects in the axial direction and the diameter thereof expands to, thereby, absorb the kinetic energy of the piston until the piston stops.
(H) Japanese Unexamined Patent Publication (Kokai) No. 63-190909 discloses another type of damper for a rodless power cylinder. The damper (an external damper) or the shock absorber in this publication is fitted to the outer wall of the tube using fitting brackets. The external damper receives the external moving body at piston stroke ends to absorb the kinetic energy of the external moving body and the piston.
(I) Japanese Unexamined Patent Publication (Kokai) No. 7-269514 discloses an arrangement of cylinder gasket interposed between the inner surface of the bore and an insert portion of the end member which is inserted into the bore. The cylinder gasket in this publication is fitted into a groove formed on the periphery of the insert portion of the end member. A bulging portion is formed on the bottom of the groove at the portion facing the inner seal band in order to press the cylinder gasket to the seal band with higher pressure than the other portions of the cylinder gasket.
However, the rodless power cylinders in the publications (A) through (I) have various disadvantages.
For example, in the publication (A), the inner seal band of a thin metal belt is pressed against the flat inner surface of the bore in order to obtain a face-to-face contact between the seal band and the inner surface. Therefore, the roughness of the bore surface and the surface of the inner seal band must be kept small in order to obtain a good seal performance. Therefore, the surfaces of the bore and the inner seal band must be machined to a high accuracy. This increases the manufacturing cost of the rodless power cylinder.
Further, since the rodless power cylinder in the publication (B) uses an elastomer seal band having a large thickness instead of a thin metal seal band, the groove deep enough to accommodating the thick elastomer seal band must be formed on the slit-side inner surface. This causes the thickness of the wall of the tube to increase and make it difficult to reduce the height (the thickness) of the tube even if a non-circular flat bore is used.
The rodless power cylinder in the publication (C) uses a tube having a circular cross section bore. Therefore, it is difficult to reduce the height of the tube. Further, the rodless power cylinder in this publication uses a thin metal inner seal band formed as an arc of a circle having a center on the longitudinal axis of the tube. Since this seal band is guided by guide rollers, the seal band is flattened when it is guided by the rollers. Therefore, the seal band is deflected by the rollers when it contacts the rollers. This lowers the durability of the seal band.
Further, since both of the contact surfaces of the recess receiving the seal band and the surface of the seal band are curved, the curvatures of both contact surfaces must strictly match each other in order to obtain a good seal performance. This requires a higher accuracy in machining the surfaces of the recess and the seal band. In addition, when the curved seal band is used, it is difficult to accurately predict the deflection of the seal band caused by the internal pressure. Therefore, it is difficult to estimate the seal performance precisely when designing the seal band. These problems make it difficult to apply the seal band in the publication (C) to the actual rodless power cylinder from the practical viewpoint.
The rodless power cylinder in the publication (D) uses a tube having a circular cross section. Therefore, it is also difficult to reduce the height of the tube. Further, the sealing capability of the seal band in this publication is determined by the contact pressure between the edges of the seal band and the slit-side inner surface, i.e., determined by the amount of the deflection of the seal band. Further, since the amount of the deflection of the seal band varies in accordance with the curvature of the slit-side inner surface, and the curvature of the slit-side inner surface varies in accordance with the diameter of the tube, the desired deflection of the seal band for achieving a maximum sealing capability must be newly calculated in order to use a tube of a different diameter.
The piston packing used in the publication (E) seals the internal pressure by the outer lip pressed against the inner surface of the bore only by the internal pressure. Therefore, when the internal pressure is low, the sealing capability of the piston packing becomes insufficient.
Further, in the piston packing of the publication (F), the bridges connecting the outer lip and the inner lip adversely affect the sealing capability of the piston packing when the internal pressure is high. When the internal pressure is high, the force exerted on the outer lip to press the outer lip against the inner surface of the bore becomes high. However, in the piston packing of this publication, since a part of this force is received by the bridge and the force to press the outer lip against the inner surface becomes insufficient. This causes insufficient sealing capability when the internal pressure is high.
The publication (G) discloses the arrangement of the damper for the piston. Though the piston is smoothly stopped by the damper in this publication, the external moving body connected to the piston by the coupling member (a yoke) itself is not stopped. Therefore, when the piston hits the damper at its stroke end, a large bending moment is exerted on the yoke by the momentum of the external moving body.
When the external damper in the publication (H) is used, the momentum of the external moving body can be absorbed by the external damper and the bending moment exerted on the yoke becomes smaller. However, since the momentum of the external moving body is large, a large shock-absorbing capability is required for the external damper or the shock absorber. This causes an increase in the manufacturing cost of the rodless power cylinder. Further, even if an external damper having an enough shock-absorbing capacity is used, a large noise is generated when the external moving body hits the external damper.
In the rodless power cylinder of the publication (I), the bulging portion is provided on the bottom surface of the groove to press the cylinder gasket against the seal band with a larger force. However, due to the elasticity of the cylinder gasket, the cylinder gasket does not closely contact with the bottom of the groove at both sides of the bulging portion. This causes the leak of the fluid through both sides of the bulging portion.
Further, since the portion of the cylinder gasket corresponding to the position of the seal band is pressed against the seal band by a larger force, the permanent deformation of the gasket becomes large at this portion. This causes the deterioration of the cylinder gasket.
Further, in order to form the bulging portion on the groove bottom of the insert portion, the shape of the die used for casting the end member becomes complicated. This also causes the increase in the manufacturing cost of the rodless power cylinder.